1. Field of the Invention
The invention relates in general to a liquid crystal display, and more particularly to a driving circuit into which pulse width modulation control circuits are integrated for a liquid crystal display.
2. Description of the Related Art
Referring to FIG. 1, the structure of a conventional liquid crystal display 100 is illustrated. The liquid crystal display 100 includes a video signal processor 102, a timing control circuit 104, a power supply circuit 106 and a liquid crystal display (LCD) module 108.
The LCD module 108 includes a data driving circuit, a scanning driving circuit, and a pixel matrix (not shown in FIG. 1). In order to convert a received video signal, for example a composite video baseband signal (CVBS) signal, to RGB data for the data driving circuit, the video signal processor 102 requires operating voltages of +5V, +3.3V, and +7.5V, for example. The +5V voltage is provided by an external power source and is used to provide the +3.3V voltage additionally through the conversion of the +5V voltage by a voltage regulator. The timing control circuit 104 is used for controlling the data driving circuit and scanning driving circuit in order for the pixel matrix to display video frames. The scanning driving circuit requires operating voltages of +15V and −10V so as to output scanning signals to the pixel matrix. The +15V voltage is defined as a highest level of voltage for scanning signals, VGH and the −10V voltage is defined as a lowest level of voltage for scanning signals, VGL.
Since the liquid crystal display 100 requires operating voltages of +7.5V, −10V, and +15V in addition to operating voltages of +5V and +3.3V, the power supply circuit 106 includes three DC-to-DC converters for supplying the voltages of +7.5V, −10V, and +15V, correspondingly. The liquid crystal display 100 therefore requires a PWM control circuit 110 to drive the three DC-to-DC converters (not shown in FIG. 1).
During a power supply process of the conventional liquid crystal display 100, the PWM control circuit 110 outputs PWM control signals to the power supply circuit 106 in arbitrary sequence. In response to the startup of the liquid crystal display 100, the power supply circuit 106 simultaneously outputs operating voltages of +15V and −10V for powering the scanning driving circuit as well as other operating voltages for powering logic circuits. If the application of the operating voltage of +15V to the scanning driving circuit is earlier than that of the operating voltages for powering logic circuits, an unexpected bulk current would be generated in a short time in the scanning driving circuit. In such situation, the scanning driving circuit would be damaged.
In addition, unexpected symbols would display during the startup of the liquid crystal display 100 if the operating voltages for powering logic circuits and operating voltages of +15V and −10V for powering the scanning driving circuit are provided at the same time. In response to the operating voltages for powering the scanning driving circuit, the scanning driving circuit generates scanning signals to the pixel matrix while the video signal processor 102 has not outputted pixel data to the data driving circuit, thus resulting in the display of unexpected things, such as unexpected symbols.
Accordingly, the structure of the conventional liquid crystal display 100 has the following disadvantages. In terms of costs, the liquid crystal display 100's requirement of at least three integrated circuits, for example, the video signal processor 102, timing control circuit 104, and PWM control circuit 110, increases the production costs. In terms of stability, the design using the liquid crystal display 100 would damage the scanning driving circuit or results in the display of unexpected things in some circumstances.